1. Field of the Invention
The present invention relates to a zoom optical system for use in an optical unit or a like device, and more particularly to a compact zoom optical system having a large zoom ratio and operable to desirably correct chromatic aberration, as well as an image pickup device incorporated with the zoom optical system, and a digital apparatus loaded with the image pickup device.
2. Description of the Related Art
In recent years, digital cameras are rapidly spread, and are widely used not only as means for inputting images into a personal computer, but also as a tool for keeping photos, as well as conventional silver halide cameras. As the digital cameras are widely spread, a demand for a compact and high-function zoom lens unit having a large zoom ratio or a wide angle is increased. Also, since the number of pixels of an image sensor is being increased year by year, there is a demand for high optical performance. Further, there is a demand for high optical performance, as compared with a conventional arrangement, in an apparatus such as a cam recorder primarily used in recording moving images, considering a still image photographing function and a demand for compatibility with high definition devices.
As a zoom lens system for satisfying the aforementioned demands, there is known e.g. a positive-dominant zoom lens unit comprising, in the order from the object side: a first lens group unmovable in zooming and having a positive optical power; a second lens group having a negative optical power and movable along an optical axis for zooming; a third lens group movable along the optical axis to keep the position of image plane at a fixed position in zooming; and a fourth lens group having a positive optical power and adapted to perform a focusing operation (see e.g. Japanese Unexamined Patent Publication No. 2005-43630 or D1).
As a lens unit suitable for a cam recorder, there is known a positive-dominant zoom lens unit comprising, in the order from the object side: a first lens group unmovable in zooming and having a positive optical power; a second lens group having a negative optical power and movable along an optical axis for zooming; a third lens group unmovable in zooming; and a fourth lens group movable along the optical axis to keep the position of image plane at a fixed position in zooming (see e.g. Japanese Unexamined Patent Publication No. Hei 8-248317 or D2).
Generally, if a large zoom ratio is attempted to be secured in a zoom lens unit, while retaining the size of the zoom lens unit, optical powers of the lens surfaces of the zoom lens unit may be unduly increased, which may degrade various aberrations. A certain degree of improvement may be obtained concerning Seidel aberration by increasing the number of aspherical surfaces, using a lens material having a large refractive index, or providing a like measure. However, it is difficult to obtain improvement on chromatic aberrations such as on-axis chromatic aberration or magnification chromatic aberration, unless a diffraction element such as a DOE (Diffractive Optical Element) or a special material such as fluorite is used, the number of lens elements is increased, or a like measure is provided. In particular, reducing second-order spectra of on-axis chromatic aberration at a telephoto end, or second-order spectra of magnification chromatic aberration at a wide angle end is an important task to perform, not to mention correction of first-order chromatic aberration.
For instance, D1 discloses reducing residual chromatic aberration by using a medium having a property that dispersion is changed in an optical axis direction. Use of the medium having the above property, however, may be disadvantageous in the aspect of productivity or cost. D2 discloses reducing residual chromatic aberration by properly setting dispersion values and partial dispersion ratios of the lens groups. However, the zoom ratio obtainable by the arrangement of D2 is at most about eight times, which is not a sufficiently large zoom ratio. Also, the first lens group requiring a large beam diameter includes a certain number of lens elements made of a low dispersion material such as fluorite, which is disadvantageous in the aspect of cost. It may be effective to increase the number of lens elements in order to secure latitude in correcting chromatic aberration, or reduce the refractive powers of the lens elements in order to reduce the aberration amount. The above measures, however, may be disadvantageous in the aspect of cost or size.